The invention relates to diffusion bonding. More particularly, the invention relates to the diffusion bonding of alloys having oxides which are of low solubility in their respective alloys. Further, the invention relates to the diffusion bonding of such alloys when at least the surfaces to be bonded may have the fine grain structure associated with superplastic forming properties.
The combined use of superplastic forming and diffusion bonding (SPF/DB) offers the potential to manufacture lighter and less expensive aircraft structures than those made by conventional means. It is particularly attractive for sheet metal structures because part and fastener counts could be reduced, thereby significantly decreasing assembly labor. Also the fabrication of structures to near-net shapes using SPF/DB technology can improve material utilization and reduce machining time and costs.
The application of SPF/DB to titanium alloys has been well demonstrated but this is not the case for advanced high strength aluminum alloys. Although impressive SPF behavior has already been demonstrated for aluminum alloys, such as 7475, and work has begun on developing superplastic properties for Al-Li alloys, the diffusion bonding technology for these materials is lagging. A simple and cost-effective diffusion bonding technique compatible with SPF technology could significantly advance the use of aluminum structures.
In diffusion bonding, flattening of the abutting surfaces is necessary in order to achieve intimate interfacial contact. Metals like titanium, which have surface oxides that easily dissolve in the metal during heating, can be readily diffusion bonded without the use of special surface preparations or interlayer diffusion aids. Unlike titanium, aluminum and its alloys (as well as, for example, zirconium and vanadium and their respective alloys) form insoluble oxides which do not readily dissolve during bonding and thus act as barriers to intimate metal-to-metal contact and subsequent diffusion.
Typically, aluminum has been diffusion bonded by methods which rely upon considerable deformation (up to 60%) and pressure (up to 40,000 psi) to rupture surface oxide barrier films or by methods which use interlayer materials to dissolve oxides and aid diffusion. In general, such methods are not compatible with the constraints imposed by SPF technology or the mechanical property requirements of a high performance structure.
For example, practical limitations set by production equipment dictate that SPF pressures probably should be limited to 1000 psi and perhaps should be much lower. In addition, other variables important to SPF, such as starting microstructure, dwell time, forming rate, dynamic recrystallization, and post heat treatment must be considered.
Aluminum has also been diffusion bonded by removing the surface oxide layers by sputtering or other suitable techniques in a hard vacuum or reduced pressure inert gas environment in order to prevent the oxide layer from being formed again before bonding. However, pressures below 10.sup.-9 Torr must be maintained in order to keep the oxide layer from forming again almost instantly in a hard vacuum and pressures of approximately 10.sup.-6 Torr in an inert gas environment are desirable. In other words, using these techniques, the cleaned surfaces can not be exposed to air prior to bonding. It is generally believed that cleaning techniques such as abrading, chemical etching or dissolving the oxide by the use of fluxes, if carried out in a vacuum or low pressure inert gas environment, to preserve the oxide cleaned surface, present problems in controlling removal of oxides from the work, etching solutions or the process chamber.
It is the principal object of the invention to provide a method for diffusion bonding alloys having oxides of low solubility.
It is another object of the invention to provide a method for diffusion bonding aluminum alloys wherein at least the surface layers have the grain structure associated with superplastic forming.
It is still another object of the invention to provide a method of diffusion bonding useful for components having superplastic properties.
It is a further object of the invention to provide a method for diffusion bonding alloys having oxides of low solubility wherein relatively simple and inexpensive surface preparation procedures can be used.
It is yet another object of the invention to provide a method of diffusion bonding wherein after surface preparation procedures are completed, the surfaces prepared have a reasonable shelf life, in air at room temperature, during which diffusion bonding may be successfully performed.
It is still another object of the invention to provide a method of diffusion bonding which requires the use of relatively low pressures to force surfaces to be bonded together.
It is a further object of the invention to provide a method of surface preparation for alloys having oxides of low solubility which is conducive to diffusion bonding.